Tri-layered selenium doped photoreceptor

ABSTRACT

A photosensitive element having a three layered photoconductive portion comprising a first layer of vitreous selenium or a vitreous arsenic-selenium alloy, a second layer comprising a vitreous selenium-tellurium alloy, and a third layer comprising a vitreous alloy of arsenic-selenium. A method of imaging the photosensitive element is also described.

United States Patent Sechak [4 1 Apr. 11, 1972 [54] TRI-LAYERED SELENIUM DOPED [56] References Cited HOTORECEPTOR P UNITED STATES PATENTS 1d, [72] Invent P sechak Penfie N Y 3,041,166 6/1962 Bardeen ..9e/1.5 [73] Assignee: Xerox Corporation, Stamford, Conn. 3,312,548 4/1967 Straughan.... ....96/1.5 Had J 26 1970 3,355,289 11/1967 Hall et a1. ..96/l.5

L 65 Primary Examiner-George F. Lesmes [2]] App No 50 2 Assistant Examiner-M. B. Wittenberg Rdated Applicafion Data Attorney-James .1. Ralabate, Donald F. Daley and Owen D.

Marjama [63] Continuation-impart of Ser. No. 583,686, Oct. 3,

1966, abandoned. [57] ABSTRACT A photosensitive element having a three layered photocon- [52] U.S. Cl. ..96/l.5, 96/1 PC, 117/217 ductive portion comprising a first layer of vitreous Selenium or E a vitreous arsenic-selenium alloy, 21 second layer comprising a 1e 0 care vitreous selenium-tellurium alloy, and a third layer comprising a vitreous alloy of arsenic-selenium. A method of imaging the photosensitive element is also described.

23 Claims, 5 Drawing Figures This application is a continuation-in-part of copending application Ser. No. 583,686 filed Oct. 3, 1966, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to xerography, and in particular, to a system utilizing a photoreceptor having a panchromatic response.

The art of xerography involves the use of a photosensitive element containing a photoconductive insulating layer which is first uniformly electrostatically charged in order to sensitize its surface. The plate is then exposed to an image of activating electromagnetic radiation such as light, X-ray, or the like which selectively dissipates the charge in the irradiated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the non-irradiated areas. The latent electrostatic image may then be developed and made visible by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. This concept was originally disclosed by Carlson in US. Pat. No. 2,297,691, and is further amplified and described by many related patents in the field.

The use of vitreous selenium, as described by Bixby in US. Pat. No. 2,970,906, remains the most widely used photoconductor in commercial xerography in that it is capable of holding and retaining an electrostatic charge for relatively long periods of time when not exposed to light, and because it is relatively sensitive to light as compared to other photoconductive materials. In addition, vitreous selenium has sufficient strength and stability to be reused hundreds or even thousands of times. Vitreous selenium, however, is susceptible to deleterious crystal growth, especially on its surface due to environmental conditions during machine operation. This crystal growth in selenium destroys its photoconductive insulating properties, and places a limit upon the effective life of a seleniurn plate. Although the spectral response of vitreous selenium is satisfactory, it is exclusively limited to the blue or bluegreen range of the visible spectrum.

US. Pat. Nos. 2,803,542 to Ullrich and 2,822,300 to Mayer et al., both teach the concept of improving the properties of vitreous selenium by the addition of elemental arsenic in amounts up to about 50 percent by weight. The addition of arsenic greatly increases the stability of selenium at elevated temperatures, In addition, arsenic concentrations greater than about percent by weight exhibit increased spectral response in the yellow-red band of the electromagnetic spectrum. Concentrations of arsenic greater than about 10 percent, however, cause a plate to retain a high residual potential with positive charging, and in addition, cause high light fatigue. The latter effect results in ghosting, which occurs after extended repetitive imaging and is characterized by a very faint, residual negative image in background areas.

Two layered photoreceptor structures have been designed to overcome some of the above noted disadvantages. These structures, for example, contain layers of selenium and selenium-tellurium alloys. U.S. Pat. No. 2,803,541 to Paris, illustrates one such patent in which improved photosensitivity is attained by using a top layer of vitreous selenium-tellurium over a layer of selenium. This structure, however, does not provide adequate abrasion resistance for automatic xerographic machine operation and also exhibits high dark discharge. Protective organic and inorganic overcoatings have been developed to provide improved abrasion resistance, but a major problem with these overcoatings is their inability to function properly through a wide range of environmental conditions. For example, at humidities greater than 85 percent relative humidity, these overcoatings become conductive and result in a loss in xerographic image resolution. In addition, below relative humidities of percent, the resistivity of these overcoatings increases, resulting in dielectric charging during xerographic machine operation which leads to an unacceptable build-up of copy background. It can therefore be seen that there exists in the art, a need for a xerographic photoreceptor having a panchromatic response while still maintaining thermal and humidity stability, good abrasion resistance, and which is not subject to fatiguing effects which result in ghostmg.

OBJECTS OF THE INVENTION It is, therefore, an object of this invention to provide a system for utilizing a composite three layered photoreceptor which overcomes the above noted disadvantages.

It is a further object of this invention to provide a system utilizing a photoreceptor having a panchromatic response.

It is another object of this invention to provide a photoreceptor which is capable of making a color print.

It is another object of this invention to provide a three layered photoreceptor having enhanced xerographic properties under varying environmental conditions.

It is another object of this invention to provide an improved three layered photoconductor.

It is yet another object of this invention to provide a method of imaging a novel photoreceptor.

SUMMARY OF THE INVENTION The foregoing objects and others are accomplished in accordance with this invention by providing a three layered structure referred to as a tri-layer photoreceptor. This trilayer photoreceptor comprises a top layer or overcoating of arsenic-selenium alloy for abrasion resistance, temperature stability, and improved dark discharge; a second layer of selenium-tellurium alloy to yield panchromatic light response; and a bulk layer of xerographic grade selenium or halogen doped arsenic-selenium. This tri-layer photoreceptor may be coated or evaporated onto any standard xerographic base by any conventional technique known to the art.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of this improved photoreceptor will become apparent upon consideration of the following disclosure of the invention, especially when taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic illustration of one embodiment of a xerographic photoreceptor as contemplated by this invention.

FIGS. 2A, 2B, 2C, and 2D represent a plot of sensitivity for various thicknesses of the top two photoreceptor layers of the tri-layer photoreceptor of the instant invention.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to the drawing, reference character 10 designates a tri-layer, xerographic photoreceptor according to this invention. This photoreceptor has a conventional electrically conductive support member 11 such as brass, aluminum, nickel, steel or the like. The support member may be of any convenient thickness, rigid or flexible, and may be in any desired form such as a sheet, web, plate, cylinder, drum, or the like. It may also comprise other materials such as metallized paper, plastic sheets coated with a thin layer of metal such as aluminum or copper iodide, or glass coated with a thin layer of chromium or tin oxide. If desired, the photoreceptor may also be formed on an electrically insulating support and electrostatically charged by xerographic processes well known to the art of xerography for photoreceptors having insulating backings. In addition, member 11 may, in some cases, be dispensed with entirely.

Layer 12 comprises a bulk layer of xerographic grade selenium such as that disclosed in US. Pat. No. 2,970,906, or a vitreous halogen doped arsenic-selenium alloy and may be of any convenient thickness. Such layers may be as thin as 10 microns, or as thick as 300 microns or more, but for most commercial applications this thickness will generally lie between about 40 to microns. The range of 40 to 100 microns is preferred in that in general, thicker layers (i.e., those approaching 300 microns and greater) show some signs of less desirable adherence to the support member than lesser thicknesses. Thin layers (i.e., about 1 to 20 microns) charged to the high level of surface potential required for optimum print properties in present xerographic machines, experience localized high dark discharge, resulting in print deletions and generally poor print quality.

Layer 13 denotes a thin layer of a vitreous selenium-telluriurn alloy having a broad range of 0.1 to 2 microns in thickness, which is provided to give a panchromatic light response (i.e., sensitive to light in the region up to about 700 nanometers). Selenium-tellurium alloys containing from about 2 to 50 percent tellurium by weight are generally satisfactory, with up to about 25 percent tellurium being sufficient to give the desired panchromatic response. Tellurium concentrations above approximately 25 percent result in an increasingly greater red response but exhibit higher light fatigue and dark discharge. If the selenium-tellurium layer is thicker than about 2 microns, high dark discharge results, while thicknesses below about 0.1 microns fail to yield a significant red response. A preferred thickness range of about 0.1 to 1.0 microns has particular utility for the reproduction of color images ad will be further described in more detail below.

Layer 14 comprises'an overcoating of a vitreous arsenicselenium alloy about 0.1 to 2.0 microns thick containing arsenic in a concentration from about 0.1 to 40 percent by weight. Arsenic in the range of about 5 to 20 percent by weight has been found to be particularly desirable in that this composition exhibits good thermal stability and abrasion resistance. If the arsenic-selenium layer is thicker than about 2 microns, the red response of the lower selenium-tellurium is substantially eliminated or reduced. If the arsenic-selenium layer is thinner than about 0.1 microns, the abrasion resistance and thermal stability provided by this layer is deleteriously affected. A preferred thickness range of about O.l to 0.6 microns has particular utility for the reproduction of color images when used with the preferred range for the seleniumtellurium layer above. Optionally, the arsenic-selenium top layer (especially layers containing arsenic concentrations greater than about percent) may be doped with about 10 to 10,000 parts per million by weight of a halogen such as chlorine or iodine in order to further improve sensitivity by reducing residual potential with positive charging. This concept is disclosed in US. Pat. No. 3,312,548.

In another embodiment of this invention, the bulk layer 12 which comprises essentially vitreous selenium, may optionally contain from about 0.1 to 0.5 percent arsenic. In addition, to lower the residual potential, halogens in amounts from about 10 to 10,000 parts per million by weight may also be added to this bulk layer.

The tri-layer photoreceptor of this invention may be prepared by any suitable technique. A typical technique includes vacuum evaporation wherein each photoconductive layer is sequentially evaporated onto its corresponding base material. In this technique, the selenium, selenium-tellurium, and arsenic-selenium layers, are each evaporated by separate steps, under vacuum conditions varying from about 10 to l0 torr. In another embodiment of this particular technique, the three photoreceptor layers are continuously vacuum evaporated, one after the other, in the same vacuum chamber without breaking the vacuum, by sequentially activating three separate sources of selenium, selenium-tellurium, and arsenicselenium.

Another typical technique includes co-evaporation, wherein the appropriate amount of material for each of the alloy layers is placed in separate heated crucibles maintained under vacuum conditions, with the source temperature of each alloy constituent being controlled so as to yield the appropriate percentage of the alloy desired. This technique is illustrated in copending application Ser. No. 566,593 filed on July 20, 1966.

Another typical method of evaporation includes flash evaporation under vacuum conditions similar to those defined in co-evaporation, wherein a powder mixture such as selenium and tellurium is selectively dropped into a heated crucible maintained at a temperature of about 400 to 600 C. The vapors formed by the heated mixture are evaporated upward onto a substrate supported above the crucible.

In all of the above methods, the substrate onto which the photoconductive material is evaporated is maintained at a temperature from about 50 to C. If desired, a water cooled platen or other suitable cooling means may be used in order to maintain a constant substrate temperature. In general, a selenium base or bulk layer thickness of about 60 microns is obtained when evaporation is continued for about I hour at a vacuum of about 5 X l0 torr at a crucible temperature of about 280 C.

US. Pat. Nos. 2,803,542 to Ullrich; 2,822,300 to Mayer et al.; 2,901,348 to Dessauer et al.; 2,963,376 to Schaffert; 2,970,906 to Bixby; and 3,077,386 to Blakney et al.; all illustrate vacuum evaporation techniques which are suitable for the formation of alloy layers of the instant invention. The crucibles which are used for evaporation of the photoreceptor layers may be of any inert material such as quartz, molybdenum, stainless steel vacuum coated with silicon monoxide, or any other equivalent materials. The selenium or selenium alloy being evaporated is maintained at a temperature between about its melting and boiling point. The photoreceptor of the instant invention exhibits a panchromatic response to visible light and thus has great flexibility with respect to its commercial utility because of its greater efficiency in respond ing to visible light. For example, for conventional line copies of light blue on white background, the instant photoreceptor produces copies of superior quality to most commercial photoreceptors. The instant photoreceptor also produces excellent black on white copies of originals which have various colored backgrounds. The preferred two top layer thickness ranges of 0.1-L0 microns for the selenium-tellurium layer and 0.1 to 0.6 microns for the arsenic-selenium layer exhibit critical thickness ranges which are particularly suitable for the making of colored prints by xerographic techniques. One such suitable technique includes the making of a color print from a positive color transparency, such as a Kodachrome (Eastman Kodak Co.) color side or transparency. The plate is first sensitized by positive corona charging followed by exposing the plate to the Kodachrome image through a red filter. The plate is then developed with cyan developer which is attracted to the unexposed or charged areas of the plate (i.e., those areas which correspond to the green and blue portions of the transparency and which were filtered out by the red filter). This portion of the developed image is then transferred to a sheet of ordinary white bond paper. The above procedure is repeated again by exposing through a green filter, and development carried out with magenta developer which is transferred to the same sheet of paper in exact register with the previous cyan image. The entire procedure is repeated a third time using a blue filter and yellow developer. This image is also transferred in register with the cyan and magenta images. The final color image is then fixed on the sheet of paper to form a permanent color copy of the color transparency. This technique can effectively reproduce colors by the proper combination of the three primary colors (i.e., red, green, and blue) or their complementaries: cyan, magenta, and yellow. The above developing technique requires that the photoconductor be sensitive to red, green, and blue light. This method is more fully described in the text Electrophotography by Schaffert, 1965 Focal Press Limited on pgs. 97-100, which is incorporated herein by reference.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples further specifically define the present invention with respect to the method of making a trilayer photoreceptor member. The percentages in the disclosure, examples, and claims, are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of making a tri-layer photoreceptor. The photoreceptor alloy elements described in the specifica- EXAMPLE n The tri-layer photoreceptor drum of Example 1 is then imaged by charging the plate to a positive potential by the method described by Carlson in U.S. Pat. No. 2,588,699. The

i {incl examples; sleniunh arsenlc, and tellurium, are an 5 corona charging unit is maintained at about 7,500 volts P Xerographlc grade (Selemum and Semc thereby charging the drum to an acceptance potential of about 99-999 P f y P f tdlul'lum 99-99 W y 1,000 volts. The charged drum is exposed to a black on white Pure) avallable from Canadlan Copper Refinel'sline copy original image contained on conventional printing a er. A l5-watt General Electrical Cool white fluorescent EXAMPLEI p p lamp No. FT8CW is used as the exposure source to form a An oxidized aluminum Substrate in form of cylindrical latent electrostatic image. The drum bearing the latent elecdrum approximately 9% inches in diameter and 15 inches long image is developed by cascadl'fg an electroscop' is placed in a vacuum chamber in direct Contact with a romp ic marking material over the photoconductive surface of the ing water cooled platen maintained at a controlled temperal5 drum The developed image transferred to a sheet of 5P ture of about 50o 60o A loading of to 3/16 inch Seleni and heat fused to make it permanent. The final image exhibits urn pellets is placed in an SiO coated stainless steel evaporagood Fsolunon and hlgh density and an excellent copy of tion boat and positioned about 10 inches below the surface of f z i th I d x 2400 C d the drum. A second SiO coated stainless steel boat containing 6 rum m a erox. we opler l/8- to 3/16-inch pellets of 25 percent tellurium and 75 per- E copy i an 3 b.lue ggi g cent selenium is placed adjacent to the selenium containing 18 T 2' 1s lgmge u SiO coated stainless boat. A third SiO coated stainless steel a is? 9 6 gh boat containing 1/8- to 3/ 16-inch pellets of 5 percent arsenic sig a papei 1s e a a erox 2400 and 95 percent selenium is placed adjacent to the second SiO Ce opler z g f ofa coated stainless steel boat. Each boat is connected directly to v] t 2: i f g copyd good a source of electrical power adaptable to control the temperai 2:? o lnage :g f i ture of the respective boats, in addition, each boat contains an 5; e f y to at orme y e smg e ayer 0 aluminum shutter which may be placed over the open surface reous memc'se of the boat to halt the deposition of each alloy at any desired M E III-VII time. The chamber is then evacuated to a vacuum of about A ri of five drums about 9% inches in diameter and 15 10 torr while the drum is rotated at about 10 RPM. The boat inches in length with an oxidized aluminum substrate are containing the selenium, with its shutter in the open position, coated according to the method set forth in Example 1. The is heated to a temperature of about 285 C. f r ab 4 first drum is coated with a 60-micron coating of selenium on minutes to form a layer of vitreous selenium about 60 microns 3 5 the oxidized aluminum and is to be further used as a standard thick on the aluminum drum. The shutter on the Sel nium control drum. Four additional drums are vacuum coated with boat is then closed, and the electrical power to the selenium the tri-layer tructure by the method set forth in Example I. boat turned off. Without breaking the vacuum, the drum These four test drums contain 60-micron coatings of selenium speed is increased to about 30 RPM and the selenium-tellurion the aluminum base followed by an overcoating of approxiurn containing boat heated to a temperature of 450 C- for mately 0.3 microns of a 75 percent selenium 25 percent telabout 6 minutes to form a selenium-tellurium coating about lurium alloy, and a final overlayer of 0.1 microns of a 5 per- 0.3 microns thick on the selenium layer. The shutter is then cent arsenic 95 percent selenium alloy. The above four closed over the selenium-tellurium boat and the power to the drums are tested for light sensitivity or drum speed and boat turned off. The shutter is opened on the arsenic-selenium print quality in comparison to the conventional vitreous boat with the drum speed remaining at 30 RPM and the arselenium control drum, and X-ray examined to determine the senic-selenium alloy evaporated onto the selenium-tellurium presence of crystallinity. All five drums are run on a modified layer by maintaining the arsenic-selenium boat at a tempera- 2400 Xerox Office Copying Machine at a ambient temperature of about 450 C. for 4 minutes to form a 0.15 micron ture of about to F. for a total of 100,000 rints to P thick layer of arsenic-selenium alloy on the selenium-tellurium determine the above mentioned properties. A summarization layer. At the end of this time the vacuum chamber is cooled to 50 of the various test properties of the standard selenium control room temperature, the vacuum broken, and the tri-layer drum and the tri-layer test drums are summarized in Table l photoconductor drum removed from the chamber. below.

TABLE I Layer 1: Layer 2: Layer 3: Selenium 75% scle- 5% arsebulk ninm-25% nic, layer, tellurium, selenium, X-ray Drum No. microns microns microns Drum speed 1 examination Print quality 2 l-tStnndard 60 None None Sylvania Sylvanin Sylvania Crystallinity With black and white line copying, selenium conbluc-grcen grccn warm at 60,000 the test drums have quality corntrol drum). lamps, lamps, white copics. parable to the selenium drum. Xerox Xerox lamp. With color subjects, the test drums Part No. Part N 0. show a 101,000% improvement 122P09. 122F518. over selenium depending upon 1.0 1.0 1.0 background color and the lamp 2(Tcst drum)... 60 0,3 0.1 2x 2x 2.5x No cvidcnco used. In addition, the trilayer ofcrystalphotoreceptor exhibit: excellent linity at copies of light blue on white images 100,000 which are superior in quality to copies. similar copies made by the sole- 3-(Tcst drum)... 60 0.3 0.1 2:: 2x 2.5x do nium control drum. Those copies sl llcst drum). 60 0.3 0.1 2x 2x 2.5x do were made using the green and 5(T0 l rlrurnl 60 0,3 0.1 2x 21: 2.5): do whitc lamps, respectively.

l l'nc drum spcctY is dclincd ns arctic oi the total spectral energy required to produce a constant potential discharge from a fixed starting potentinl comparing the test drums to the standard selenium drum. Each drum was charged to 1,000 volts positive potential under dark room conditions and discharged in separate tests with uniform exposure to the three light sources set forth undcr DRUM SPEED".

For the blue-green lamps; for a setting of 14.5 at a M inch slit width 1.00 foot candles were required for the test drums, while at a setting of f 4.5 at

Z4 inch slit width 3.50 foot candles were needed for the selenium drum.

With thc grccn lamps; with a. setting of f 4.5 at a Ms inch slit width 3.85 foot Zic inch slit width 8.50 foot candles were needed for the selenium drum.

With the warm white lamps; with a setting of i 4.5 at a inch slit width at a 9;; inch slit width 5.65 foot candles were needed for the selenium drum.

1 Print quality is a subjective rating of the acceptability of (black on white) xcrographic prints on the basis of background density, image sharpness, resolution, low density reproduction (black on white subject), and color reproduction. Color reproduction relates to the ability of a drum to copy from color on white subiccts and color on color subjects.

candles were required for the test drums, while at a setting of 4.5 at u 2.25 foot candles were required for the test drums, while at a setting of 4.5

As seen from Table I above, with warm white lightexposure lamps, the tri-layer drums are capable of producing 100,000 prints or copies which are superior in overall print quality and color response to that of the standard selenium test drum. At optimum exposure, the tri-layer drums exhibited an increase in speed of approximately twice that of standard selenium. X- ray examination of the tri-layer photoreceptor drums after 100,000 copies at ambient temperatures of 7590 F, shows no evidence of crystallinity when examined by X-ray difi'raction and fluorescence analysis. This is contrasted with a marked degree of crystallinity which occurred at ambient temperatures of 8590 F in a selenium test drum at approximately 60,000 print cycles, which required the selenium drum to be rejuvenated by polishing with a cloth pad soaked in Brasso (available from the R. T. French Co.) in order to remove the thin surface layer of grey, crystalline selenium, thereby restoring the initial xerographic print properties.

EXAMPLE VIII The plate of Example I is used to form a color print from an original image in the form of a color transparency. The plate is imaged as follows: The plate is uniformly corona charged to a positive acceptance potential of about 1,000 volts. Using a white light source, the plate is then exposed to the color transparency image through a red filter. The latent image formed by this exposure is then developed by cascading cyan developer particles over the plate. The developed cyan particulate image is then transferred to a receiver sheet of white bond paper. The above imaging procedure is then repeated on the plate by exposing through a green filter, followed by developing with magenta developer and transferring the magenta particulate image to the same receiver sheet in exact register with the previous cyan image. The developing step is repeated a third time using a blue filter and developing with yellow developer particles. The yellow particulate image is then transferred to the same receiver sheet in exact register with the two previous images. The composite color image contained on the receiver sheet is then fixed by heat fusing to form a permanent color copy of the original color transparency. A second color image is made in the form of a color transparency by repeating the above process and substituting a sheet of cellulose acetate as the receiver sheet in place of the white paper.

EXAMPLE IX A series of 36 plates having varying thicknesses for the two top photoconductive layers are made according to the method of Example I. The thickness of the selenium is kept constant at about 60 microns. The thickness of the various layers for each of these plates are tabulated in Table II below. The plates are numbered l-36, respectively.

TABLE II Arsenic- Seleniumselenium tellurium Selenium top layer middle layer base layer thickness, thickness, thickness, (microns) (microns) (microns) Plate N 0.:

1 0. 1 60 0. 0. 1 60 0. O. 1 60 0. 0. 1 60 0. 0. 1 60 0.30 0. 1 60 0.35 O. 1 60 0. 50 O. 1 60 1. 00 0. 1 60 0 0. 3 60 0. 05 0. 3 60 0. 10 0. 3 60 0. l5 0. 3 60 0. 20 0. 3 60 0. 30 0. 3 60 0. 35 O. 3 60 0, 50 0. 3 6O 1. 00 O. 3 60 0 0. 6 60 0.05 0. 6 60 0. 1O 0. 6 60 0. 15 O. 6 6O Fable Il- Continued Arsenic- Seleniumselenium tellurium Selenium top layer middle layer base layer thickness, thickness, thickness, (microns) (microns) (microns) 0.20 0. 6 '60 0. 0. 6 60 0. 0. 6 6O 0. 50 0. 6 60 1. O0 0. 6 60 l. 0 60 0. 05 1. 0 60 0. 10 1. 0 60 0. l5 1, 0 Fl] 0. 20 l. 0 60 0. 30 l. 0 60 0. 35 1. 0 PK) 0. 50 l. 0 Fl) 1. 00 1. 0 60 The plates listed in Table [I above illustrate varying thicknesses for the two top layers. For the making of a color image a sensitivity of at least 0.1 cm lerg is necessary to form satisfactory images having the necessary density and color rendition. Sensitivities less than about 0.1 cm /erg result in images having low density and weak in colors corresponding to the low sensitivity.

The tri-layer photoreceptor of the instant invention exhibits its lowest sensitivity in the green portion of the visible spectrum. As stated above, it has been found that a sensitivity of at least 0.1 cm /erg at any wavelength in either the blue, green,

or yellow-red, is necessary with available light sources in order to achieve good color rendition in making color copies. Therefore, with the lowest sensitivity occurring in the green portion of the spectrum, the sensitivity with respect to the thickness of the two top layers is measured at two wavelengths within this portion of the visible spectrum. These wavelengths are 525 nanometers and 550 nanometers respectively, and as will presently be shown, illustrate the critical thickness of the two top layers which are necessary in order to maintain a minimum sensitivity in the green region. Sensitivities greater than 0.1 cm lerg are exhibited by the instant photoreceptor in the region from 350 to 525 nanometers and from about 550 to 700 nanometers.

The spectral sensitivity (HE) is defined as the reciprocal of the incident energy required to produce a specific response. As defined in the instant application, this is the reciprocal energy necessary to produce a 25 percent discharge for a photoreceptor charged to an initial field of 16.5 volts/micron. This corresponds to a discharge of 250 volts for an initial positive charge of 1,000 volts placed on a 60-micron layer of the photoreceptor. In order to illustrate the criticality for the two top layer thicknesses for use in reproducing color images, the sensitivity in the green portion of the visible spectrum is measured at 525 nanometers and 550 nanometers for each of the plates listed in Table ll using a filtered tungsten light source. It can be seen that the plates are classified into four groupings: One grouping containing nine plates having a selenium-tellurium layer thickness of O. 1 microns with varying thicknesses for the arsenic-selenium layer; a second group comprising nine plates having a 0.3 micron selenium-tellurium layer and varying arsenic-selenium layer thicknesses; a third group contains nine plates having a selenium-tellurium layer of 0.6 microns with a varying thickness of the arsenic-selenium; and a fourth group of nine plates having a 1.0 micron layer of selenium-tellurium with varying thicknesses for the arsenic-selenium layer. Each of these four groups are plotted separately in FIGS. 2A, 2B, 2C, and 2D, respectively, with regard to the sensitivity (l/E) compared to the variation in top layer thickness for a single constant layer thickness for the selenium-tellurium layer. It can be seen that for the response in the green region (525 and 550 nanometers) that in order to maintain a sensitivity of at least 0.1, the top layer thickness should be maintained in a range from about 0.l to about 0.6 microns (see FIG. 2D). The maximum of 0.6 microns is essential in order to maintain at least a sensitivity of 0.1 to green light while the minimum thickness of 0.1 is essential to obtain good wear properties. The range of thickness for the selenium-tellurium layer is from about 0.1 to 1.0 microns.

It can be seen from the above disclosure that the tri-layer xerographic plate exhibits a panchromatic response and xerographic properties superior to that of vitreous selenium. Further, the tri-layer photoreceptor exhibits a capability for reproducing color copies. In addition, the tri-layer drum shows greater stability for long run cycle applications than vitreous selenium which has an inherent crystallization problem at relatively short cycling times.

Although specific components and proportions have been stated in the above description of the preferred embodiment of this invention, other suitable materials and procedures such as those listed above, may be used with similar results. In addition, other materials and changes may be utilized which synergize, enhance or otherwise modify the tri-layer photoreceptor. For example, the addition of small amounts of halogen dopants, and small percentages of additional alloying elements are all contemplated within the scope of this invention.

Other modifications and ramifications of the present invention would appear to those skilled in the art upon reading the disclosure. These are intended to be within the scope of this invention.

What is claimed is:

1. A composite photoreceptor member comprising:

a. a first layer of vitreous selenium,

b. a second layer comprising a vitreous seleniumtellurium alloy overlaying said selenium layer,

c. and a third layer comprising a vitreous arsenic-selenium alloy overlaying said selenium-tellurium layer, wherein said selenium-tellurium layer is about 0.1 to 1.0 microns in thickness, and said arsenic-selenium layer is about 0.1 to 0.6 microns in thickness.

2. The member of claim 1 in which the vitreous selenium layer contains a halogen dopant.

3. The member of claim 1 in which the vitreous selenium layer contains up to about 0.5 weight percent arsenic.

4. The member of claim 3 in which the vitreous layer further contains a halogen dopant.

5. The member of claim 1 in which the arsenic-selenium layer is doped with a halogen.

6. A photoconductive member comprising:

a. a conductive support,

b. a layer of vitreous selenium overlaying said support,

c. a layer of vitreous selenium-tellurium alloy about 0.1 to

1.0 microns thick overlaying said selenium layer,

d. and a layer of a vitreous arsenic-selenium alloy about 0.1 to 0.6 microns thick overlaying said selenium-tellurium layer.

7. The member of claim 6 in which the support comprises an electrically insulating material.

8. The member of claim 6 wherein the selenium-tellurium alloy contains up to about 50 percent by weight tellurium.

9. The member of claim 6 wherein the selenium-tellurium alloy contains about 25 percent by weight tellurium.

10. The member of claim 6 wherein the arsenic-selenium alloy contains up to about 40 percent'arsenic by weight.

11. The member of claim 6 wherein the arsenic-selenium alloy contains about 5 percent arsenic by weight.

12. The member of claim 6 wherein the selenium layer contains up to about 0.5 percent arsenic by weight.

13. The member of claim 12 in which the layer further contains a halogen dopant.

14. The member of claim 6 wherein the selenium layer is doped with a halogen.

15. The member of claim 6 wherein the arsenic-selenium layer is doped with a halogen.

16. A photoconductive member comprising:

a. a conductive support,

b. a layer of vitreous selenium overlaying said support,

c. a 0.1 to 1.0 micron layer of a vitreous selenium-tellurium alloy containing up to about 25 weight percent tellurium overlaying said selenium layer,

d. and a 0.1 to 0.6 micron layer of a vitreous arsenic-selenium alloy containing about 0.1 to 40 weight percent arsenic overlaying said selenium-tellurium layer.

17. The member of claim 16 in which the selenium layer is about 40 to microns thick.

18. The member of claim 16 in which the selenium layer is doped with a halogen.

19. The member of claim 16 in which the vitreous selenium layer contains arsenic in an amount of about 0.1 to 0.5 percent by weight.

20. The member of claim 19 in which the vitreous layer further contains a halogen dopant.

21. The member of claim 19 in which the vitreous selenium layer further contains a halogen dopant.

22. An imaging method comprising:

a. providing a photoconductive member comprising a conductive substrate coated with a vitreous selenium overlayer, a layer of a vitreous selenium-tellurium alloy 0.1 to 1.0 microns thick overlaying said selenium layer, and a layer of a vitreous arsenic-selenium alloy 0.1 to 0.6 microns thick overlaying said selenium-tellurium layer;

b. forming a latent electrostatic image on said member; and,

c. developing said image.

23. A method of forming a latent electrostatic image which comprises:

a. providing a photoconductive member having a conductive support, a layer of vitreous selenium overlaying said support, a layer of a vitreous selenium-tellurium alloy about 0.1 to 1.0 microns thick overlaying said selenium layer, and a layer of a vitreous arsenic-selenium alloy about 0.1 to 0.6 microns thick overlaying said seleniumtellurium layer;

b. substantially uniformly electrostatically charging said member; and

c. exposing said member to a pattern of activating electromagnetic radiation to form a latent electrostatic image. 

2. The member of claim 1 in which the vitreous selenium layer contains a halogen dopant.
 3. The member of claim 1 in which the vitreous selenium layer contains up to about 0.5 weight percent arsenic.
 4. The member of claim 3 in which the vitreous layer further contains a halogen dopant.
 5. The member of claim 1 in which the arsenic-selenium layer is doped with a halogen.
 6. A photoconductive member comprising: a. a conductive support, b. a layer of vitreous selenium overlaying said support, c. a layer of vitreous selenium-tellurium alloy about 0.1 to 1.0 microns thick overlaying said selenium layer, d. and a layer of a vitreous arsenic-selenium alloy about 0.1 to 0.6 microns thick overlaying said selenium-tellurium layer.
 7. The member of claim 6 in which the support comprises an electrically insulating material.
 8. The member of claim 6 wherein the selenium-tellurium alloy contains up to about 50 percent by weight tellurium.
 9. The member of claim 6 wherein the selenium-tellurium alloy contains about 25 percent by weight tellurium.
 10. The member of claim 6 wherein the arsenic-selenium alloy contains up to about 40 percent arsenic by weight.
 11. The member of claim 6 wherein the arsenic-selenium alloy contains about 5 percent arsenic by weight.
 12. The member of claim 6 wherein the selenium layer contains up to about 0.5 percent arsenic by weight.
 13. The member of claim 12 in which the layer further contains a halogen dopant.
 14. The member of claim 6 wherein the selenium layer is doped with a halogen.
 15. The member of claim 6 wherein the arsenic-selenium layer is doped with a halogen.
 16. A photoconductive member comprising: a. a conductive support, b. a layer of vitreous selenium overlaying said support, c. a 0.1 to 1.0 micron layer of a vitreous selenium-tellurium alloy containing up to about 25 weight percent tellurium overlaying said selenium layer, d. and a 0.1 to 0.6 micron layer of a vitreous arsenic-selenium alloy containing about 0.1 to 40 weight percent arsenic overlaying said selenium-tellurium layer.
 17. The member of claim 16 in which the selenium layer is about 40 to 100 microns thick.
 18. The member of claim 16 in which the selenium layer is doped with a halogen.
 19. The member of claim 16 in which the vitreous selenium layer contains arsenic in an amount of about 0.1 to 0.5 percent by weight.
 20. The member of claim 19 in which the vitreous layer further contains a halogen dopant.
 21. The member of claim 19 in which the vitreous selenium layer further contains a halogen dopant.
 22. An imaging method comprising: a. providing a photoconductive member comprising a conductive substrate coated with a vitreous selenium overlayer, a layer of a vitreous selenium-tellurium alloy 0.1 to 1.0 microns thick overlaying said selenium layer, and a layer of a vitreous arsenic-selenium alloy 0.1 to 0.6 microns thick overlaying said selenium-tellurium layer; b. forming a latent electrostatic image on said member; and, c. developing said image.
 23. A method of forming a latent electrostatic image which comprises: a. providing a photoconductive member having a conductive support, a layer of vitreous selenium overlaying said support, a layer of a vitreous selenium-tellurium alloy about 0.1 to 1.0 microns thick overlaying said selenium layer, and a layer of a vitreous arsenic-selenium alloy about 0.1 to 0.6 microns thick overlaying said selenium-tellurium layer; b. substantially uniformly electrostatically charging said member; and c. expOsing said member to a pattern of activating electromagnetic radiation to form a latent electrostatic image. 